In the 1960s, the skies above the United States were patrolled by agents of the apocalypse. Air Force B-52 Stratofortresses circled the North American continent, 24 hours a day, cradling two megabombs in their bellies. Those B-53 bombs each weighed 10,000 pounds. Were one to drop on the White House, a nine-megaton yield would destroy all life out into suburban Maryland and Virginia.

Out at the Energy Department's Pantex Plant near Amarillo, Texas, the last of America's B-53s is in storage. Come Tuesday, it will be dissected: The 300 pounds of high explosives will be separated from its enriched uranium heart, known as a "pit." The pit will be placed into a storage locker at Pantex, where it will await a final, highly supervised termination.

First brought into the U.S. nuclear stockpile in 1962, the B-53 was so big because it was so dumb. With poor precision mechanisms for finding a target -- "Its accuracy was horrendous," Kristensen says. And it was designed to burrow deep. The dumb bomb wouldn't destroy [a target] so much as it would destroy everything remotely near it, leaving -- literally -- a smoldering crater.